1. Field of the Invention
This invention pertains to a CMOS voltage controlled oscillator (VCO), and more particularly to a CMOS VCO having a voltage-to-current converter and an active MOS load operating in the triode region to provide improved performance characteristics.
2. Description of the Related Art
VCOs generate an output voltage whose frequency varies in step with an input voltage and are used in a variety of applications where maintaining sequential operations by a clocked pulse is required. Various types of VCOs have been proposed over the years. VCOs comprised of bipolar junction transistors have been used to generate output frequencies in 5 to 10 MHz range. Later, as the demand to use higher speed integrated circuits increased, CMOS based VCOs were designed and constructed to operate at considerably higher frequencies of up to around 300 MHz.
One type of higher speed CMOS VCO that has been proposed employs a plurality of inverters comprised of CMOS transistors connected in a ring configuration. Another similar type uses a plurality of delay circuits, each comprised of a differential pair of MOS transistors, connected in a ring configuration. The delay time in each of the delay circuits is controlled by adjusting the amount of current supplied to each of the delay circuits.
Although these CMOS VCOs were an improvement over their predecessors using bipolar junction technology, they presented certain problems as well. For example, variations in the power source voltage typically resulted in changes in the delay time which, in turn, resulted in variations in the frequency of operation, making the VCO unsuitable for low jitter applications.
An improvement along these lines was proposed in U.S. Pat. No. 5,673,008 which provides a CMOS VCO that is immune to variations in power voltage. This VCO employs a plurality of differential input/output delay cells arranged in a ring configuration. However, the structure of this CMOS VCO is such that it is not able to generate a small logic swing on the order of 0.5 V to enable it to generate very high frequencies. In particular, the load transistors in the delay cells are conventionally configured to operate in the saturation region which limits its performance characteristics.